Physics is built on measurement. To measure a quantity is to compare it against a fixed, agreed reference called a unit, and report how many times that unit fits into the quantity. A physical quantity always has two parts: a numerical value and a unit. Writing the length of a room as just "5" is meaningless; $5\\,\\text{m}$ carries information. In symbols, a measured quantity $Q$ is written as $Q = n\\,u$, where $n$ is the numerical value and $u$ the unit. Crucially, $n$ and $u$ are inversely related: if you switch to a smaller unit, the number grows. The same length is $1\\,\\text{m}$ or $100\\,\\text{cm}$, so $n_1 u_1 = n_2 u_2$.
Quantities split into two groups. Fundamental (base) quantities are chosen to be independent of one another and cannot be expressed in terms of others — length, mass and time are the classic three. Derived quantities are built from the base ones by multiplication or division: speed is length divided by time, force is mass times acceleration, and so on. Their units are derived units assembled from base units.
A complete, self-consistent set of base units plus the derived units built from them is a system of units. Historically several competed — CGS (centimetre, gram, second), FPS (foot, pound, second) and MKS (metre, kilogram, second). The modern global standard is the SI system (Système International d'Unités), which rests on seven base units:
- metre (m) for length, kilogram (kg) for mass, second (s) for time;
- ampere (A) for electric current, kelvin (K) for thermodynamic temperature;
- mole (mol) for amount of substance, candela (cd) for luminous intensity.
Two supplementary units handle angles: the radian (rad) for plane angle and the steradian (sr) for solid angle. To avoid clumsy numbers, SI uses prefixes that scale a unit by powers of ten — kilo ($10^{3}$), mega ($10^{6}$), giga ($10^{9}$), and downward milli ($10^{-3}$), micro ($10^{-6}$), nano ($10^{-9}$). So $1\\,\\text{km} = 10^{3}\\,\\text{m}$ and $1\\,\\mu\\text{m} = 10^{-6}\\,\\text{m}$.
SI also fixes conventions that keep scientific writing unambiguous: unit symbols are never pluralised (write $5\\,\\text{kg}$, not $5\\,\\text{kgs}$), no full stop follows a symbol unless it ends a sentence, a space separates the number from the unit, and symbols named after people are capitalised (N for newton, Pa for pascal) while the spelled-out name stays lower case.
Deeper Insight — why the kilogram was redefined: Until 2019 the kilogram was a lump of platinum-iridium kept in Paris, and the world's mass standard could literally lose atoms. The SI base units are now defined through fixed values of fundamental constants — the second via the caesium atom's transition frequency, the metre via the speed of light, and the kilogram via Planck's constant $h$. The lesson for you is conceptual: a good unit must be reproducible anywhere and unchanging in time, which is exactly why nature's constants make better rulers than physical objects.